Graduate school has a reputation for being a tremendous time-sink. I'm currently married with kids, and I have a fairly busy extracurricular schedule. Is it common for married people - or anyone, actually, with significant external time constraints - to not finish their graduate work, specifically due to conflicting obligations?
I've seen it work both ways: some people realize that they have a limited amount of time to work as a result of their external commitments, and therefore make themselves super-productive during the hours they are able to do research. I think, to some extent, that most of the people with severe external obligations fall into this class.
For a sizable minority, however, the balancing act proves too difficult—although this is often a function of a mismatch in expectations between the advisor and the graduate student. If you believe this could be a problem for you, you should definitely talk with your advisor; if the outcome is unsatisfactory, you should also consider speaking with your thesis committee and the "graduate officer" of your department.
In general, I think that if you are a productive graduate student, your advisor would be willing to work out suitable accommodations for your schedule.
The people who I've seen with considerable external commitments who have still gotten through in a reasonable amount of time all boil down to one thing: They treat grad-school like a job. From 9 to 5 (or 10 to 6, 11 to 7, whatever works for their schedule) they're working. No, they're not available for the infinitely many distractions of life, any more than someone at an office is. They pack a tremendous amount of productivity into that time, and then when they need to be with their family, they're with their family.
Those at least are the people who got through it sane. I think the other major trait is to recognize that, because you have a life, this might take a little longer, and not burn yourself out trying to hammer things through to the point of exhaustion.
I had a similar dilemma (mine was whether to become a parent during grad school), and what solved it for me is the realization that, if all goes according to plan, my post-PhD life won't be too different from my grad student life. I'd like to stay in academia, and all the current difficulties will remain (e.g. short-term contracts/grants, traveling, unevenly distributed workload throughout the year, etc.). This realization was one of those: "Aha!" moments for me. If it doesn't work out now, it won't be a viable option for the future either. That being said, I do try to keep a 9-5 schedule, and I don't think it would have worked out well if my partner wasn't as involved in parenting as I am.
Perhaps working in industry is different, and it really is easier to have kids. In any case it might be an idea to ask yourself whether you think your obligations in grad school would really be all that different from what they are at your current job.
I think the answers given by aeismail and EpiGrad are correct, however, I just want to add one particular point, the one of traveling. Indeed, if it's true that many jobs from 9 to 5 can also include some traveling duties, it's particularly true when doing a PhD (and later on during the whole academic life). In this case, I don't think that marriage is a problem, but other external time obligations, such as a baby or an activity requiring some presence can pose problem.
I've known a case of a PhD student who couldn't attend a conference overseas in its entirety due to religious reasons: the conference was ending on a Friday, and this student needed to be home every Saturday. He couldn't attend entirely for the same reason a PhD school, and I think it's a real problem, because these kind of events are also there to socialize with peers, and going there just for a limited time window does not help.
Similarly, I've known a case where a speaker could only attend one day at a conference, the one where he was giving his talk, as he couldn't be far from home more than that, because he had to take care of his baby.
I'm not saying that an academic life means no private life, but in some cases, it can require some flexibility, and an external time obligation might imply to limit oneself, for instance by considering only "local" conferences.
I agree with previous answers: graduate school is a full-time job, and should be treated as such. I think it is possible to do so while keeping a healthy marriage life, though it might take more time than usual.
Just to complete the perspective, let me give my personal experience. I have completed my M.Sc. in parallel of working a full-time job (which was really a 24/7 kind of work). Although it was M.Sc and not PhD, it was research-based rather than classes-based and same rules apply: school is a full-time job. Graduating took me 6 years rather than the common 2-year program, but I did it! Don't get me wrong - it wasn't easy at all: I was working on weekdays and studying on weekends, with very little time to do other stuff (if at all). However, I know quite a few people who did the same. The key to success is only to have a strong will. Kids and marriage are a large time-consuming, but certainly not as much as another job.